// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE-BSD file. See the AUTHORS file for names of contributors.

// SPDX-License-Identifier: Apache-2.0
// Copyright 2020, Intel Corporation

#ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_
#define STORAGE_LEVELDB_UTIL_RANDOM_H_

#include <stdint.h>

namespace leveldb
{

// A very simple random number generator.  Not especially good at
// generating truly random bits, but good enough for our needs in this
// package.
class Random {
private:
	uint32_t seed_;

public:
	explicit Random(uint32_t s) : seed_(s & 0x7fffffffu)
	{
		// Avoid bad seeds.
		if (seed_ == 0 || seed_ == 2147483647L) {
			seed_ = 1;
		}
	}
	uint32_t Next()
	{
		static const uint32_t M = 2147483647L; // 2^31-1
		static const uint64_t A = 16807;       // bits 14, 8, 7, 5, 2, 1, 0
		// We are computing
		//       seed_ = (seed_ * A) % M,    where M = 2^31-1
		//
		// seed_ must not be zero or M, or else all subsequent computed values
		// will be zero or M respectively.  For all other values, seed_ will end
		// up cycling through every number in [1,M-1]
		uint64_t product = seed_ * A;

		// Compute (product % M) using the fact that ((x << 31) % M) == x.
		seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
		// The first reduction may overflow by 1 bit, so we may need to
		// repeat.  mod == M is not possible; using > allows the faster
		// sign-bit-based test.
		if (seed_ > M) {
			seed_ -= M;
		}
		return seed_;
	}
	// Returns a uniformly distributed value in the range [0..n-1]
	// REQUIRES: n > 0
	uint32_t Uniform(int n)
	{
		return Next() % n;
	}

	// Randomly returns true ~"1/n" of the time, and false otherwise.
	// REQUIRES: n > 0
	bool OneIn(int n)
	{
		return (Next() % n) == 0;
	}

	// Skewed: pick "base" uniformly from range [0,max_log] and then
	// return "base" random bits.  The effect is to pick a number in the
	// range [0,2^max_log-1] with exponential bias towards smaller numbers.
	uint32_t Skewed(int max_log)
	{
		return Uniform(1 << Uniform(max_log + 1));
	}
};

} // namespace leveldb

#endif // STORAGE_LEVELDB_UTIL_RANDOM_H_
